Solid-state optical amplifier having an active core and doped cladding in a single chip

ABSTRACT

A solid-state optical amplifier is described, having an active core and doped cladding in a single chip. An active optical core runs through a doped cladding in a structure formed on a substrate. A light emitting structure, such as an LED, is formed within and/or adjacent to the optical core. The cladding is doped, for example, with erbium or other rare-earth elements or metals. Several exemplary devices and methods of their formation are given.

PRIORITY CLAIM

This application is a divisional of, and claims the benefit of, U.S.patent application Ser. No. 15/344,730, filed Nov. 7, 2016, which ishereby incorporated in its entirety by this reference.

BACKGROUND

The following is related generally to the optical components used inoptical communication networks, and specifically to optical devices thatcan amplify optical signals.

Erbium-Doped Fiber Amplifiers (EDFAs) or Praseodymium-doped fiberAmplifiers (PDFAs) are widely deployed in optical networks, in the 1550nm or 1310 nm wavelength windows, respectively. FIG. 1 illustrates themultiple optical components that are typically included in a prior artEDFA or PDFA. The optical power from the pump laser light source 102 iscombined with the input signal 101, by a wavelength-divisionmultiplexing (WDM) coupler 104. The combined input signal and pump laserlight then passes through a section of fiber 103 that has been dopedwith erbium or praseodymium ions in its core. The pump laser lightexcites the erbium or praseodymium ions embedded in the erbium-doped (orpraseodymium-doped) fiber 103 to a higher energy level. The opticalinput signal 101 then induces stimulated emission and is thereforeamplified to create the output signal. However, amplified spontaneousemission (ASE) noise is also generated simultaneously, and creates noiseon top of the amplified input signal 101. Thus the output signal 106consists of an amplified input signal, as well as the ASE noisecomponent. An isolator 105 is located after the erbium-doped orpraseodymium-doped fiber 103. This isolator 105 is intended to preventthe back scattering power out of the downstream optical fiber and othercomponents from re-entering the EDFA or PDFA. This unwanted backscattering power would otherwise be amplified, and would thereforeinterfere with the EDFA's (or PDFA's) normal characteristics andperformance. Also shown in FIG. 1 is a pump laser monitoring port 107.

FIG. 2A illustrates the principle of operation of an Erbium-Doped FiberAmplifier (EDFA). The figure shows three energy levels, labeled E₀, E₁,and E₂, of Er³⁺ ions in silica glass. Each of the energy levels aresplit into multiple levels or bands, via the Stark splitting process, asdescribed in “Erbium-Doped Fiber Amplifiers, Fundamentals andTechnology”, Chapters 8 and 9, P. C. Becker, N. A. Olsson, and J. R.Simpson, Academic Press, 1999, for example. The difference between anytwo of the energy levels is labeled with the wavelength (or wavelengthrange) of the photons that correspond to that energy level transition.The upward arrows indicate the wavelengths at which the EDFA can bepumped, in order to excite the erbium ions to the indicated higherenergy level. For example, a 1480 nm pump laser can be used to excitethe erbium ions from the E₀ level to the E₁ level, whose life time isvery long, on the order of 10 msec, such that population inversionbetween the E₀ level and the E₁ level is suitable for stimulatedemission. The downward arrow from E₁ to E₀ represents the wavelengthrange of photons emitted due to spontaneous and stimulated emission,amplifying the input signal. Because the E₁ and E₀ energy levels aresplit into bands, a range of wavelengths can be amplified, shown in FIG.2 as 1520-1560 nm.

Similarly, a 980 nm pump laser can be used to excite the erbium ionsfrom the E₀ level to the E₂ level. The ions that have been raised to theE₂ level quickly transition to the E₁ level, via a non-radiativespontaneous emission process. The transition of these ions from the E₁level to the E₀ level results in amplification of input signals in the1520-1560 nm range, via stimulated emission. For a variety of reasons,pumping at 980 nm is more efficient than pumping at 1480 nm.

Pump sources of wavelengths lower than 980 nm can also be used,including visible light. FIG. 2B shows a more complete view of theenergy levels of Er³⁺ ions. The wavelength scale corresponds to thewavelength being emitted when Er³⁺ ions transit from a given energylevel to the ground state. The energy level diagram illustrates thatEr³⁺ ions can absorb light from approximately 1500 nm, down to less than400 nm. Lower-wavelength pump sources (i.e., lower than 980 nm) excitethe erbium ions to higher energy levels than are shown in FIG. 2A, butthe overall process for amplifying the input signal via stimulatedemission is similar. Also, for input signals of different wavelengthranges, different rare-earth elements, or other materials, includingvarious metals, may be used, with correspondingly different pump sourcerequirements. For example, to amplify input signals in the 1310 nmwavelength range, praseodymium ions have energy level transitions thatare in the appropriate wavelength range. Amplifying an input signal ofvisible light is also physically feasible, as long as the pumpingwavelength is shorter than that of the input light, and suitable dopantsare used. Broadband sensitizers can also be used, as an aid to theenergizing of the dopant material. (More detail on broadband sensitizerscan be found, for example, in “Broadband Sensitizers For Erbium-DopedPlanar Optical Amplifiers: Review”, A. Polman and F. van Veggel, Journalof the Optical Society of America B, Vol. 21, Iss. 5, May 2004.).

As has been occurring with cell phones, more and more components arebeing squeezed into individual optical modules, with the same limitedvolume, in order to save space, and also to upgrade the performance ofnetwork control centers. Fiber splicing between separate fiber opticcomponents is cumbersome, and also occupies space. It is thereforehighly desirable to integrate multiple optical components into a singlepackage. Prior art EDFAs and PDFAs, as illustrated in FIG. 1, typicallymake use of separate WDM coupler and isolator components, and alsoincorporate a length of doped fiber (typically 10 to 20 meters inlength) with bend radius limitations. Further, the pump laser source iseither an external component, or else it must be integrated into theEDFA housing. These considerations limit the size (and cost) reductionsthat can be achieved with typical prior art optical amplifiers that arebased on the use of doped fibers. Semiconductor optical amplifiers(SOAs) can provide size advantages, but typically have performancelimitations, due to higher levels of amplified spontaneous emission(ASE) noise, as well as a variety of non-linear behaviors. It istherefore desirable for the multiple component elements of the dopedfiber (or more generally, doped waveguide) optical amplifier to bephysically integrated into as few components or elements as possible,but without the performance limitations of semiconductor opticalamplifiers.

SUMMARY

An optical amplifier includes a substrate and an optical core formed onthe substrate that provides an optical path from an optical input to anoptical output. One or more light emitting structures are formed overthe substrate within and/or adjacent to the optical core. One or morecladding layers are formed on the substrate within which the opticalcore and light emitting structure are located, the cladding layer (orlayers) having an index of refraction that is lower than an index ofrefraction of the optical core. The one or more cladding layers aredoped with one or more dopant elements or materials. Electrical contactsare connected to the light emitting structure, whereby in response to anapplied voltage differential the light emitting structure illuminates atleast a portion of the one or more doped cladding layers. The dopantelements or materials emit light within a first wavelength range whenilluminated by the light emitting structure by light of a wavelengththat is shorter than that of the first wavelength range.

In other examples, an optical amplifier includes a substrate and anoptical core formed on the substrate providing an optical path from anoptical input to an optical output. One or more light emittingstructures is formed over the substrate within and/or adjacent to theoptical core. Cladding along the optical core and light emittingstructure has an index of refraction that is lower than an index ofrefraction of the optical core. The cladding is doped with one or moredopant elements or materials. Electrical contacts are connected to thelight emitting structure, whereby in response to an applied voltagedifferential the light emitting structure illuminates at least a portionof the doped cladding. The dopant elements or materials emit lightwithin a first wavelength range when illuminated by the light emittingstructure by light of a wavelength that is shorter than that of thefirst wavelength range.

A method of forming an optical amplifier includes growing a lightemitting epitaxial layer on a first substrate and forming a patternedoptical core and a light emitting structure within and/or adjacent tothe optical core from the light emitting epitaxial layer. One or morecladding layers are deposited on the patterned optical core. Thecladding layer or layers have an index of refraction that is lower thanan index of refraction of the optical core and is doped with one or moredopant elements or materials. The dopant elements or materials emitlight within a first wavelength range when illuminated by light of awavelength that is shorter than that of the first wavelength range. Afirst set of electrical connections contacting the light emittingstructure is fabricated on the one or more cladding layers. A secondsubstrate is formed over the first set of electrical connections and thefirst substrate is subsequently removed from the patterned optical coreformed on it. A second set of electrical connections is fabricated overa surface exposed by removing the first substrate, the second set ofelectrical connections contacting the light emitting structure.

A further method of forming an optical amplifier includes growing alight emitting epitaxial layer on a first surface of a substrate andforming from the light emitting epitaxial layer a patterned optical coreand a light emitting structure within and/or adjacent to the opticalcore. Cladding is deposited along the optical core and light emittingstructure. The cladding has an index of refraction that is lower than anindex of refraction of the optical core and is doped with one or moredopant elements or materials. The dopant elements or materials emitlight within a first wavelength range when illuminated by light of awavelength that is shorter than that of the first wavelength range. Afirst set of electrical connections is fabricated over the lightemitting structure, the first set of electrical connections contactingthe light emitting structure. A second set of electrical connections isfabricated on or penetrating through a second surface of the substrate,the second set of electrical connections electrically contacting thelight emitting structure.

Various aspects, advantages, features and embodiments are included inthe following description of exemplary examples thereof, whichdescription should be taken in conjunction with the accompanyingdrawings. All patents, patent applications, articles, otherpublications, documents and things referenced herein are herebyincorporated herein by this reference in their entirety for allpurposes. To the extent of any inconsistency or conflict in thedefinition or use of terms between any of the incorporated publications,documents or things and the present application, those of the presentapplication shall prevail.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an Erbium-Doped (or Praseodymium-Doped) FiberAmplifier (EDFA or PDFA, respectively), used to amplify an opticalsignal.

FIG. 2A illustrates the principles of operation of an Erbium-Doped FiberAmplifier (EDFA).

FIG. 2B shows a more complete view of the energy levels of Er³⁺ ions.

FIG. 3A shows the waveguide layout and routing of one embodiment.

FIG. 3B illustrates an exemplary process for the fabrication of theembodiment such as shown in FIG. 3A.

FIG. 4 shows a cross-section view of the embodiment shown in FIG. 3A.

FIG. 5 shows an enlarged view of one portion of the embodiment shown inFIG. 3A.

FIG. 6A shows typical LED epitaxial structure.

FIG. 6B illustrates the energy bands of the LED epitaxial structureshown in FIG. 6A.

FIG. 7 shows another embodiment, with light-emitting regions surroundingand interspersed with the waveguides.

FIG. 8 shows a cross-section view of the embodiment shown in FIG. 7.

FIG. 9 shows a cross-section view of an additional embodiment, as alsoshown in FIG. 11A, incorporating LED epitaxial structures within thewaveguide core(s).

FIG. 10 shows an enlarged view of one portion of the embodiment shown inFIG. 9.

FIG. 11A shows a top view of the embodiment shown in FIG. 9, depictingthe waveguide layout and routing, as well as the inclusion of a waveplate.

FIG. 11B illustrates an exemplary process for the fabrication of theembodiments such as shown in FIG. 11A.

FIG. 12A shows a detailed view of the wave plate such as can be used inembodiments like those shown in FIG. 11A, including the coupling oflight from and to the adjacent waveguides.

FIG. 12B illustrates use of a polarization rotator, utilizing an offsettwo-core structure, surrounded by a common cladding.

FIG. 13 shows another embodiment, utilizing a polarization rotator atthe input port, for reducing polarization dependent loss.

FIG. 14 provides a perspective view of a polarization rotator such as isused in the embodiment shown in FIG. 13.

DETAILED DESCRIPTION

The techniques described in the following provide for the multiplecomponent elements of a doped fiber (or more generally, doped waveguide)optical amplifier to be physically integrated as a single opticalamplifier chip, but without the performance limitations of semiconductoroptical amplifiers.

FIG. 3A illustrates a first set of exemplary embodiments that have anactive core 305 that emits pump light and also acts as a waveguide corefor the input signal, with the active core being embedded within acladding that has been doped with a rare-earth element, or anothersuitable dopant material. The input signal is being carried by an inputoptical fiber 301, and is coupled to an end edge 303 of a planarwaveguide that is formed by an active core 305 and its surroundingcladding 306. The active core 305 has a light emitting structure such asa Light Emitting Diode (LED) epitaxial structure within, based onmaterials such as InGaAsP, InGaAs or InGaN compound semiconductormaterials (as described below), whose index of refraction (about 2.1 to3.8) is generally higher than that of the cladding 306. (The followingdescription will refer to LED embodiments for simplicity, but, moregenerally, this can be taken more generally as a light emittingstructure.) The cladding 306 is doped with a rare-earth element such asErbium, Praseodymium, Thulium, Ytterbium, Neodymium, or theircombinations. The dopant material may also be a metal, or some othermaterial that can emit light at the appropriate wavelength or wavelengthrange. The total thickness (marked by T in FIG. 4) of all epitaxiallayers in the active core 305 is a few microns. Erbium is used as anexample of the doped rare-earth element in the following description,without losing its generality and applicability to other rare-earthelements, or to other dopant materials. More generally, the cladding'sdopant elements or materials are chosen to emit light within a firstwavelength range when illuminated by the LED or other light emittingstructure, by light of a wavelength that is shorter than that of thefirst wavelength range, where in many common applications the firstwavelength range is a subset of the range of 400 nm to 3,000 nm.Although erbium is used in much of the following discussion as arepresentative dopant material, other dopant materials are within thescope of the present invention, including, but not limited to, otherrare-earth elements and other metals.

As shown in the example of FIG. 3A, the active core 305 is looped in aclockwise direction, through a substantial number of turns, as indicatedby arrows 311 a through 311 f, within a planar chip 300. The active core305 then makes a reversing turn at 312 a, and then progresses through aseries of counter-clockwise loops, as indicated by arrows 312 b through312 d, until it reaches its end point at the chip edge 317, which iscoupled to an output optical fiber 328. The total effective length ofthe active core may be as long as tens of centimeters, or even tenmeters. Because the Refraction Index (RI) of the active core 305 ishigher (either significantly higher or just slightly higher) than thatof the cladding 306 (the cladding index typically ranging from about1.45 to about 2.01), the optical power is confined within the activecore 305. The active core 305 has cross-sectional dimensions on theorder of one micron, or even sub-micron, for operating wavelengths from1300 to 1600 nm. The dimensions of the active core 305 along the path ofits loops can vary, based on the demands and requirements for effectiveoptical power confinement. For instance, stronger power confinement isneeded at a sharp turn such as 312 a, and thus the core dimensionsshould be slightly larger there. In the straight sections, as indicatedby 311 a through 311 d, it is desirable for more optical power to bespread from the active core 305 into the cladding 306 to inducestimulated emission of the erbium ions embedded there (as explainedlater), and thus the width (marked by W in FIG. 4) of the active cores401 through 407 should be smaller, down to sub-micron.

FIG. 4 shows a cross-section view of the embodiment of FIG. 3A, taken atthe A-A “cut line”. The active core (305) loops of FIG. 3A are shown inFIG. 4 as cross-sections 401 through 407, and the surrounding cladding306 of FIG. 3A is shown by 421 and 422 in FIG. 4. The thin layer orlayers composed of the active core and the cladding is bonded to asubstrate 410 (as explained later in the description of the fabricationmethod). Positive “point contact” electrodes, as exemplified by 441 and442, are dispersively deposited onto the p-side of the active LED coreloop 305, and are interconnected to a positive terminal electrode 445for supplying positive voltage to the active LED core loop 305.Similarly, negative “point contact” electrodes, as exemplified by 451and 452, are dispersively deposited onto the n-side of the active LEDcore loop 305, and are interconnected to negative terminal electrode455, to supply a negative voltage to the active LED core loop 305.

The cross-sections of active cores 401 through 407 are surrounded by anupper cladding 421 and a lower cladding 422, which are comprised of ahost material (preferably in an amorphous state or nano-clusteringstate), such as SiO₂ (RI about 1.45), silicon-rich SiO_(x) having Sinanocrystals within as broadband sensitizers, SiO_(x)N_(y) (RI rangingfrom about 1.45 to about 2.01), or even polymers, doped with erbium ions(or other rare-earth ions). The Refraction Index (RI) of the claddingmaterial can therefore range from about 1.45 to about 2.01. The activecores 401 through 407 emit pump light at a wavelength that is around 980nm or 1480 nm, or another wavelength in the visible light andultraviolet (UV) ranges that will excite Er³⁺ ions (or other dopantmaterials) embedded in the silica (or SiO_(x)N_(y)) host material.Visible wavelengths as well as near-infrared and UV wavelengths are alsofeasible as long as they can pump the Er³⁺ ions (or other dopantmaterials) to the desired energy levels. In the case where the pumpingwavelength is much shorter than the signal wavelength, broadbandsensitizers may be required, to be embedded (or to co-exist) with theoptically active dopant material within the cladding, in order toenhance the emissions from the optically active dopants. (See, forexample, in “Broadband Sensitizers For Erbium-Doped Planar OpticalAmplifiers: Review”, A. Polman and F. van Veggel, Journal of the OpticalSociety of America B, Vol. 21, Iss. 5, May 2004.) If other rare-earthelements or other dopants are used, for amplification of different inputsignal wavelengths, then the pump light wavelength must be compatiblewith the type of dopant used, as well as the intended input signalwavelength range. Furthermore, amplifying an input signal of visiblelight is also physically feasible, as long as the pumping wavelength issignificantly shorter than that of the input light and suitable dopantsas well as sensitizers are used in the cladding.

The active core can be composed of direct bandgap semiconductormaterials such as III-V compound semiconductors, including InGaAs (RI ofabout 3.4), InGaAsP (RI of about 3.6), InGaN and AlInGaN (RI of about2.1), or other semiconductor materials that have a direct bandgap forefficient light emission. Thus, the RI of the cladding material cangenerally range from about 1.45 to about 2.01, while the RI of theactive core can generally range from about 2.1 to about 3.8. Throughproper selection of the active core material and the cladding material,both high index-contrast waveguides (with strong confinement) and lowindex-contrast waveguides (with weak confinement) can be constructed.

The input signal light propagating along the active core loop 305 inFIG. 3A forms a fundamental mode, with its evanescent field penetratinginto the cladding 306 to induce stimulated emission of the excited Er³⁺ions in the cladding, such that the stimulated emission photons arecoupled back into the active core loop 305, amplifying the fundamentalmode. Because the photon energy of the signal light is less than thebandgap energy of the LED's quantum well (QW) material, or the activelayers in a hetero-junction LED structure, the absorption loss of signallight in the active core 305 is negligible. Furthermore, optical losscaused by the cladding is very low, because the dielectric cladding hasvery little absorption of the input signal light, as well as the pumplight.

FIG. 5 is an enlarged view of inset 340 from FIG. 3A. Negativeelectrodes 501 are dispersed on the top of the active core sections 511through 515, and are electrically interconnected by conductive trace504. The cladding sections 530 through 535 are interleaved into the corearray formed by core sections 511 through 515. The up-arrow shown inactive core section 512 and the down-arrow shown in active core section513 indicate the direction of signal light propagation as anillustration. The electrical field distribution 521 of the fundamentalmode propagating in active core section 512 has the majority of itsoptical power confined in the core, but its evanescent field 523penetrates into the cladding 532, to induce stimulated emission of theexcited Er³⁺ ions, thus enhancing its field strength. The signal lightthat propagates in the downward direction (as indicated by arrow 518) inactive core section 513 will do the same. Because the index of thecladding material is lower than that of the active core, the coupling ofAmplified Spontaneous Emission (ASE) noise from the erbium ions in thecladding into the fundamental mode is very limited.

FIGS. 6A and 6B illustrate one example of a typical LED epitaxialstructure and its corresponding energy bands. (In these figures, thePosition axes correspond to downward in FIG. 3B or 4, and the upwardaxis in FIG. 6A corresponds to a lateral direction.) In FIG. 6A, anetching stop layer 602 having a composition such as InGaP is initiallygrown using Metal-Organic Chemical Vapor Deposition (MOCVD) or MolecularBeam Epitaxy (MBE), on the top of a wafer substrate 601. Then an N-typecompound semiconductor layer 605 is grown, followed by a quantum well(QW) or multiple QW layer 607. Finally, a P-type layer 609 is added ontop of the QW layer 607. Depending on the desired emission wavelength,the composition of the compound material is chosen. For emission around980 nm, QW compound material having a composition of(In_(x)Ga_(1-x))_(0.5)As_(0.5) is selected, and a compound ofcomposition (In_(x)Ga_(1-x))_(0.5)(As_(y)P_(1-y))_(0.5) is chosen for1480 nm emission. For lower wavelength pump light, AlInGaP can be usedfor 630 nm emission, InGaN can be used for emission in the 420 nm to 480nm range, and AlInGaN can be used for emission in the 370 nm to 420 nmrange. (For more detail refer, for example, to Chapter 1 of “DiodeLasers and Photonic Integrated Circuits”, L. Coldren, S. Corzine, and M.Mashanovitch, 2^(nd) Edition, Wiley Publishing, 2012, and Chapter 4 of“Introduction to Solid-State Lighting”, A. Zukauskas, M. Shur, and R.Gaska, Wiley Publishing, 2002.) FIG. 6B shows the corresponding energyband gap of the epitaxial structure illustrated in FIG. 6A. Modernmultiple-QW LEDs have high quantum efficiency of emission in the visiblelight range, between 65% and 95%.

In order to fabricate the material structure of the embodiments such asthose shown in FIGS. 3A, 4, and 5, an exemplary sequence of fabricationprocesses is illustrated in FIG. 3B:

-   -   1. Select a semiconductor wafer such as GaAs, which has a        lattice constant that is similar to the subsequent epitaxial        layers.    -   2. Grow an etching stop layer having a composition such as        InGaP, onto the wafer, by either Metal-Organic Chemical Vapor        Deposition (MOCVD) or Molecular Beam Epitaxy (MBE).    -   3. Then grow, via sequential MOCVD or MBE processes, an LED        epitaxial structure (about 1 to 4 microns in total thickness).        One example of a typical LED structure is illustrated in FIG.        6A.    -   4. Use photolithography and then chemical vapor etching or wet        etching to create a spiral active core loop 305, indicated in        FIGS. 3A, 4, and 5.    -   5. Utilize either chemical vapor deposition or physical        sputtering to deposit an erbium-doped SiO2 layer (with thickness        of about 10 microns), shown as the lower cladding layer or        layers 422 in FIG. 4, on the top of the patterned active core        loop.    -   6. Fabricate the positive electrodes, extending through the        lower cladding to contact the p-side of the active core loop,        using a sequence of lithography, etching, and metal deposition.        Metal traces are used to interconnect all of the positive        electrodes. Since these metal traces can dissipate optical        power, their width should be minimized.    -   7. Either epoxy or metallurgically bond a new substrate, which        has a thermal expansion coefficient close to those of the        cladding and active core (semiconductor wafers are preferred),        to the lower cladding. The new substrate may be of an        electrically conductive or non-conductive material.    -   8. Use wet chemical etching to remove the original wafer (GaAs        in this example, as described in step 1, above), until the        etching is stopped at the etching stop layer indicated in step        2.    -   9. Deposit the top or upper erbium-doped cladding layer or        layers 421 (as shown in FIG. 4), using the same process as        indicated in step 5.    -   10. Create the negative electrodes and their electrical        interconnections for the n-side LED active core, using similar        processes as indicated in step 6.    -   11. Dice the finished wafer into individual optical amplifier        chips, which have dimensions of about 10 mm×10 mm.        The sequence processing sequence and recited details of FIG. 3B        is meant as an exemplary process and not meant to be exhaustive        of the possible variations. It should be noted that although        referred to in FIG. 3B (and also below in FIG. 11B) as “steps”        for expository purposes, in actual processing any of these        listed process phases may involve multiple sub-steps or,        conversely, several “steps” may be combined in a single        processing operation, so that “step” in not meant to be limiting        on how the process is performed in practice.

In practice, the fabrication of the positive and negative “pointcontact” electrodes, so that they adhere properly to the narrow activecore loop 305, through cladding layer 306 (as shown in FIG. 3A, and alsoshown in more detail in FIGS. 4 and 5), is feasible but may bechallenging in some process flows. FIG. 7 illustrates another set ofembodiments that avoids “point contact” electrodes, with its emittingregion and the waveguide core being defined or created from the same LEDepitaxial structure, fabricated from a wafer. Firstly, the core loop 705and pump emission regions (as indicated by hatched areas 711, 712 and713, for example) are created from a planar LED epitaxial structure.(The non-hatched areas are here covered by cladding, as indicated byreference numbers 814 and 816 in FIG. 8.) Electrode pads 721 through 728are fabricated on top of the pump emission regions 711, 712, and 713,respectively, for electrical wire bonding. The interconnecting metalsection, or trace (shown as 751 in the figure), is intended to ensureeffective current spreading over the emission region 711. Then theerbium-doped cladding is deposited to enclose the core loop 705,excluding the pump emission regions 711, 712, and 713. The emissionregions can be interposed within the core loops as well or arrangedarbitrarily (for example, emission regions 715 and 716) as long as theyare in close proximity to the cladding area, for good coupling of thepump light. Because the pump emission photons have energies that are notmore than the bandgap energy of the LED structure within the core 705,absorption by the core is limited. Therefore the pump photons can comeacross or traverse the core several times before they reach the claddingareas which are located farther away from the emission regions. If somephotons are absorbed by the core 705, then they are likely to be“recycled”, to be re-emitted from the core. In order to maximize theextraction of pump light from the pump emission regions, the interfacebetween the emission regions and the cladding areas may be shaped ortextured, or given a structure, as depicted graphically by items 770,771, and 772.

FIG. 8 is a cross-section view from FIG. 7, taken at the B-B cut line.Items 831 and 833 indicate the multiple core cross-sections, and items814 and 816 are their corresponding top-side erbium-doped claddings,situated on top of core sections 831 and 833. The pump emission regionsare indicated by 811, 812, and 813, and their corresponding top-sideelectrode pads are indicated by 826, 827, and 823, respectively. Theirbottom-side electrode pads are indicated by 836, 837, and 838,respectively. The lower cladding layer 840 is bonded to a substrate 810.Pump light emitted from emission region 811 first enters the claddinglayer at the location indicated by 846, to excite the erbium ionswithin. The unabsorbed photons then penetrate the next core 836 to reachthe next section of the cladding layer 847, and so on. Pump emissionsfrom the other two emission regions 812 and 813 behave in similarfashion. Mirrors or reflective coatings 851 and 853 may be applied tothe outer edges of emission regions 811 and 813, respectively, toprevent pump light from leaking out of the amplifier chip 800.

The fabrication processes for the embodiments illustrated in FIGS. 7 and8 can be similar to the processes described above for the embodimentsillustrated in FIGS. 3A, 4, and 5, and as illustrated in FIG. 3B.

FIGS. 9 and 11A show yet another set of embodiments, which can also helpto avoid the fabrication difficulties associated with the use of“point-contact” electrodes. FIG. 11A shows the top view of opticalamplifier chip 1100, and FIG. 9 is a cross-section view, taken at theC-C cut line of FIG. 11A. FIG. 10 is an enlarged view of inset 920 fromFIG. 9. As also shown in FIGS. 4 and 8, T represents the total thicknessof the LED epitaxial layers, and W is the width of the active waveguidecores. Typically T is on the order of a few microns, and W is about onemicron. Thus T/W (referred to as the aspect ratio) is therefore on theorder one, or slightly larger. However, in the embodiment shown in FIGS.9, 10, and 11A, T is about 5 to 10 microns and W is about 1 micron orsub-micron. The total thickness T includes N-type epitaxial layers 901grown from, or on top of the original wafer substrate 900 (as explainedin the description of fabrication processes, below), the light-emittingquantum well or active layers in the hetero-junction LED structurerepresented by 902, and the P-type layers 903, as well as thecorresponding layers within the active cores 931 and 932. Thus T istypically one order of magnitude larger than W, resulting in the aspectratio being on the order of ten. Furthermore, based on the refractionindex for typical LED structure layers, the index of the QW is generallythe highest index within the structure, and the index decreases withdistance from the QW. Thus there is a weak optical confinement towardthe QW layer. With this sort of geometry and index distribution, theactive cores 931 and 933 and their respective claddings 947 and 940, actlike slab waveguides, which have a power density distribution of thefundamental mode in an elliptic shape, as depicted graphically by item1012 in FIG. 10. The power density distribution has its peak power pointaligned with the QW 1002, with its long axis lying along the active coreslab 1001 (i.e., the X-axis shown in FIG. 10).

Note that in FIGS. 9 and 10, the N-type epitaxial layers represented by901, and the lower half of the active cores 931 and 933, have been grownor formed on top of the substrate 900. The substrate material may beeither electrically conductive or non-conductive. If the substrate 900is electrically conductive, then the electrical connections to theN-type epitaxial layers may be made to the back side of the substrate,as shown in FIG. 9. However, if the substrate 900 is non-conductive,then it will be necessary to use an etching process (or some otherappropriate process) to create vias or through-holes that pass throughthe substrate material, in order to establish electrical connections tothe lower side of the N-type epitaxial layers.

The power distribution 1012 of the fundamental mode can be said to carrytwo electric polarizations, referred to as the TE mode (the electricfield primarily along the X-axis of FIG. 10) and the TM mode (theelectric field primarily along the Y-axis). It is to be expected thatthe coupling coefficients of stimulated emission from the erbium ions inthe cladding layers 947 and 940, into the active cores 931 and 933, willbe slightly different between the TE mode and the TM mode. The gains (inoptical amplification) for the TE mode and the TM mode are thus slightlydifferent as well. The amplification therefore has a polarizationdependency, which could be a negative effect for many applications. Toremedy this, as shown in FIG. 11A, a 180-degree phase shift (alsoreferred to as a half-wavelength shift) wave plate 1108 may be insertedinto a slot 1102, that has been added at approximately the middle pointof the core loop 1105. Thus, a signal that enters the optical amplifierchip at the input port 1101, and is amplified as it propagates to theoutput port 1120, will have its two polarizations “swapped” for thesecond half of the amplification loop (i.e., from the wave plate 1108 tothe output port 1120). The exact position of the wave plate 1108 alongthe core loop 1105 can be optimized to minimize the polarizationdependence of the optical amplifier chip 1100. Additionally, a waveplate may similarly be introduced into other embodiments described hereas needed or desired.

Although the wave plate 1108 is only a few hundred microns in thickness,it causes coupling loss between the incoming waveguide and the outgoingwaveguide as shown in FIG. 12A, which is an enlarged view of the inset1130 from FIG. 11A. As illustrated in FIG. 12A, the coupling loss can bereduced by modifying the waveguide geometry on either side of wave plate1108 (shown in FIG. 12A as item 1205). In the embodiment shown in FIG.12A, tapering the waveguide core-ends 1211 and 1212 of the incomingwaveguide 1221 and the outgoing wave guide 1222, respectively, resultsin the coupling loss being less sensitive to the axial distance (markedby s) of the slot 1202 of the wave plate 1205, between the two waveguideends 1211 and 1212 (refer, for example, to “Highly Efficient CouplingSemiconductor Spot-Size Converter with an InP/InAlAsMultiple-Quantum-Well Core”, N. Yoshimoto, et al., Applied Optics, Vol.34, No. 6, February 1995). The cladding material that lies between thetapered waveguide core ends 1211 and 1212, and the wave plate 1205,improves the coupling of light into and out of the wave plate. Otherbeam expansion methods for waveguide modes (refer, for example, to“Arrayed Waveguide Collimator for Integrating Free-Space Optics onPolymer Waveguide Devices”, J. S. Shin, et al., Optics Express 23801,Vol. 22, No. 20, October 2014), prior to entering the wave plate 1205,are also within the scope of the present embodiments.

FIG. 12B shows an example (see “Silicon Photonic Circuit withPolarization Diversity”, H. Fukuda, et al., Optics Express 4872, Vol.16, No. 7, March 2008) illustrating an embodiment of a polarizationrotator, utilizing an off-set two-core structure, surrounded by a commoncladding 1257. An input signal with random polarization, having beenlaunched into the input port 1250 of the first core 1251, is coupledinto two modes, denoted by TE and TM. Each mode is rotated bybirefringence as it passes through the second core, which has arefraction index (RI) that is less than that of the first core. Given aproper length of the second core 1256, the TE mode is rotated to the TMmode, and the TM mode is rotated to the TE mode, at the output port 1258of the first core 1251. A polarization rotator such as the prior artembodiment shown in FIG. 12B can be used to replace the wave platestructure illustrated in FIGS. 11A and 12A.

FIGS. 13 and 14 show another set of embodiments, utilizing anothermethod for managing the polarization dependence of the optical amplifierchip 1100 that was described above, and shown in FIG. 11A (as well asFIGS. 9 and 10). FIG. 13 is identical to FIG. 11A, except that the waveplate 1108 in FIG. 11A is eliminated, and instead a polarization unitingor combining structure 1308 is added between the input port 1101 and thebeginning of the active core loop 1105, where such a polarizationuniting or combining structure can similarly be used in otherembodiments presented here as needed or desired. (A top view of thepolarization uniter 1308 is shown in FIG. 14. Signal light that islaunched into the input port 1101 can be decomposed into twopolarization modes, a TE-like mode (electric field primarily along they-axis) and a TM-like mode (electric field primarily along the x-axis)).Both modes propagate along the waveguide section 1401 until they aresplit by a polarization splitter 1402 into two paths 1407 (carrying theTM mode) and 1408 (carrying the TE mode). The TE mode is converted to aTM mode by a polarization rotator 1409, as exemplified by FIG. 12B. Thenthe two TM modes in paths 1407 and 1408, respectively, are combined by acoupler 1415, and coupled into the beginning of active core loop 1105.Thus, only the TM mode propagates and is amplified through the fullactive core loop 1105. In similar fashion, the polarization uniter 1308can also be used to convert an input signal with random polarization,into a signal that carries primarily a TE mode.

The embodiments shown in FIGS. 9, 10, and 11A, as well as the similarembodiment shown in FIGS. 13 and 14, include LED (or, more generally,other light emitting structure) emitting regions, similar to the LEDemitting regions shown in the embodiment of FIGS. 7 and 8. It is worthmentioning that these LED emitting regions 911, 912 and 913 (as shown inFIG. 9) are used to increase the pump power, but they can be eliminated(as in the embodiment shown in FIG. 3A), since now the active cores 931and 933 emit by themselves. In the embodiment of FIGS. 9, 10, and 11A(as well as the embodiment of FIGS. 13 and 14), the positive currentspreading metal layer 908 can therefore be directly deposited on top ofthe P-type layers of the LED structure(s), including active cores 931and 933 and LED emitting regions 911, 912 and 913. The current spreadinglayer 908 may also be applied on top of the cladding areas. Since thiscurrent spreading layer 908 is relatively far away from the powerdistribution of the fundamental mode, it doesn't appreciably dissipatesignal power. The negative current spreading metal layer 921 isdeposited on the back side of the original wafer substrate 900, if thesubstrate is a conductive material. The positive electrode connectionwire 915 and the negative electrode connection wire 925 are bonded tobonding pads 909 and 924, respectively, to provide voltage and currentto the LED structure. If the substrate 900 is a non-conductive material,then vias or through-holes must be formed through the substrate so thatnegative electrode connections can be made to the N-type epitaxiallayers, as described above.

In order to fabricate the material structure of the embodiments o shownin FIGS. 9, 10, and 11A, as well as the similar embodiments shown inFIGS. 13 and 14, an exemplary sequence of fabrication processes isillustrated in FIG. 11B:

-   -   1. Select a semiconductor wafer such as GaAs, which has a        lattice constant that is similar to the subsequent epitaxial        layers.    -   2. Grow N-type layers, a QW, and then P-type layers onto the        wafer, by either Metal-Organic Chemical Vapor Deposition (MOCVD)        or Molecular Beam Epitaxy (MBE), having an LED structure that is        similar to the illustration in FIG. 6A.    -   3. Use photolithography and then chemical vapor etching or wet        etching to create a spiral active core loop 1105 (and LED        emitting regions), its corresponding grooves, and a slot 1102        for later insertion of a wave plate, as shown in FIGS. 9, 10,        and 11A.    -   4. Utilize either chemical vapor deposition or physical        sputtering to deposit an erbium-doped SiO₂ layer to the grooves,        until this cladding material is level to the N-type layers of        the active core loop.    -   5. Deposit a metal layer for positive current spreading, and        then its bond pad(s).    -   6. Deposit a metal layer to the back side of the wafer substrate        for negative current spreading, and then its bond pad(s). (As        discussed above, if the wafer substrate is a non-conductive        material, then vias or through-holes must be formed through the        substrate material, to establish electrical connections between        the negative bond pads and the N-type epitaxial layers.)    -   7. Dice the finished wafer into individual optical amplifier        chips, which have dimensions of about 10 mm×10 mm, and then        insert the wave plate 1108.        As with FIG. 3B above, FIG. 11B is meant as an example and        variations and alternatives may be employed; and the use of        “steps” in FIG. 11B is for expository purposes.

It should be noted that the top and bottom cladding layers in theembodiments described in FIGS. 3A, 4, and 5, and FIGS. 7 and 8, are notrequired for the embodiments described in FIGS. 9, 10, and 11A, andFIGS. 13 and 14. This is because the active cores and their surroundingcladdings (of the embodiments shown in FIGS. 9, 10, and 11A, and FIGS.13 and 14) act like slab wave guides. Thus there is no need to removethe original substrate wafer and then bond a new wafer to the LEDepitaxial structure.

The foregoing detailed description has been presented for purposes ofillustration and description. It is not intended to be exhaustive or tolimit the invention to the precise form disclosed. Many modificationsand variations are possible in light of the above teaching. Thedescribed embodiments were chosen in order to best explain theprinciples involved and their practical application, to thereby enableothers skilled in the art to best utilize the various embodiments andwith various modifications as are suited to the particular usecontemplated. It is intended that the scope of the invention be definedby the claims appended hereto.

It is claimed:
 1. An optical amplifier, comprising: a substrate; anoptical core formed on the substrate providing an optical path from anoptical input to an optical output; one or more light emittingstructures formed over the substrate within and/or adjacent to theoptical core; one or more cladding layers formed on the substrate withinwhich the optical core and light emitting structure are located, the oneor more cladding layers having an index of refraction that is lower thanan index of refraction of the optical core, and wherein the one or morecladding layers are doped with one or more dopant elements or materials;and electrical contacts connected to the light emitting structure,whereby in response to a voltage differential applied thereto the lightemitting structure illuminates at least a portion of the one or moredoped cladding layers, wherein the dopant elements or materials emitlight within a first wavelength range when illuminated by the lightemitting structure by light of a wavelength that is shorter than that ofthe first wavelength range.
 2. The optical amplifier of claim 1, whereinthe light emitting structure includes a light emitting diode (LED)structure.
 3. The optical amplifier of claim 2, wherein LED structureincludes one or more quantum wells.
 4. The optical amplifier of claim 1,wherein the light emitting structure is formed within part of at least aportion of the optical core.
 5. The optical amplifier of claim 1,wherein the light emitting structure is formed on the substrate adjacentto at least a portion of the optical core and separated therefrom by theone of more cladding layers.
 6. The optical amplifier of claim 1,wherein the first wavelength range is a subset of a range of 400 nm to3,000 nm.
 7. The optical amplifier of claim 1, wherein the one or moreof the dopant elements or materials includes one or more rare-earthelements or metals.
 8. The optical amplifier of claim 7, wherein the oneor more of the rare-earth elements includes erbium.
 9. The opticalamplifier of claim 7, wherein the one or more of the rare-earth elementsinclude one or more of praseodymium, thulium, ytterbium, or neodymium.10. The optical amplifier of claim 1, wherein as formed on the substratethe optical core has a spiral-type of geometry that loops back on itselfsuch that adjacent portions of the optical path run in oppositedirections.
 11. The optical amplifier of claim 1, wherein the one ormore cladding layers include a broadband sensitizer.
 12. The opticalamplifier of claim 1, further comprising: a wave plate in the opticalpath from the optical input to the optical output.
 13. The opticalamplifier of claim 1, further comprising: a polarization rotator in theoptical path from the optical input to the optical output.
 14. Theoptical amplifier of claim 1, further comprising: a polarization unitingstructure between the optical input and at least a portion of theoptical core, wherein the polarization uniting structure includes apolarization rotator such that polarizations of light incident at theoptical input are combined into a single polarization.
 15. A method offorming an optical amplifier, comprising: growing a light emittingepitaxial layer on a first substrate; forming a patterned optical coreand a light emitting structure within and/or adjacent to the opticalcore from the light emitting epitaxial layer; depositing one or morecladding layers on the patterned optical core, the one or more claddinglayers having an index of refraction that is lower than an index ofrefraction of the optical core, and wherein the one or more claddinglayers are doped with one or more dopant elements or materials, whereinthe dopant elements or materials emit light within a first wavelengthrange when illuminated by light of a wavelength that is shorter thanthat of the first wavelength range; fabricating a first set ofelectrical connections on the one or more cladding layers, the first setof electrical connections contacting the light emitting structure;forming a second substrate over the first set of electrical connections;subsequently removing the first substrate from the patterned opticalcore formed thereon; and fabricating a second set of electricalconnections over a surface exposed by removing the first substrate, thesecond set of electrical connections contacting the light emittingstructure.
 16. The method of claim 15, further comprising: prior tofabricating the second set of electrical connections, forming anadditional cladding layer or layers over the surface exposed by removingthe first substrate, the second set of electrical connections beingformed over the one or more additional cladding layers, the one or moreadditional cladding layers having an index of refraction that is lowerthan an index of refraction of the optical core, and wherein the one ormore additional cladding layers are doped with one or more rare-earthelements or other dopant materials.
 17. The method of claim 15, furthercomprising: forming an etching stop layer on the first substrate priorto growing the light emitting epitaxial layer, the light emittingepitaxial layer being grown on the etching stop layer.
 18. The method ofclaim 15, wherein the light emitting structure includes a light emittingdiode (LED) structure.
 19. The method of claim 18, wherein LED structureincludes one or more quantum wells.
 20. The method of claim 15, whereinthe light emitting structure is formed within part of at least a portionof the optical core.
 21. The method of claim 15, wherein the lightemitting structure is formed on the first substrate adjacent to at leasta portion of the optical core and separated therefrom by the one or morecladding layers.
 22. The method of claim 15, wherein the firstwavelength range is a subset of a range of 400 nm to 3,000 nm.
 23. Themethod of claim 15, wherein the one or more of the dopant elements ormaterials includes one or more rare-earth elements or metals.
 24. Themethod of claim 23, wherein the one or more of the rare-earth elementsinclude erbium.
 25. The method of claim 23, wherein the one or more ofthe rare-earth elements include one or more of praseodymium, thulium,ytterbium, or neodymium.
 26. The method of claim 15, wherein thepatterned optical core and light emitting structure are formed in anetching process.
 27. The method of claim 15, wherein the one or morecladding layers are deposited in a chemical vapor deposition process.28. The method of claim 15, wherein the one or more cladding layers aredeposited in a sputtering process.